A typical capacitor is composed of a series of capacitor packs, each of which includes wound layers of metal foil separated by a dielectric layer. Electrodes are connected to the foil layers and the electrodes of the various packs are connected together for final connection to the terminal of the capacitor.
The processing of electrical apparatus, such as a capacitor, is extremely important in providing the desired efficiency and reliability. In processing the capacitor, a dielectric liquid is introduced into the interior of the casing and acts to impregnate the dielectric layers. In the past, the processing has been carried out by initially placing a group of capacitors in an autoclave, and heating the capacitors to a temperature, generally in the range of 180.degree. F. to 240.degree. F., while drawing a vacuum on the autoclave to remove air and water vapor from the interior of the capacitor. In some cases, in the past, an individual vacuum line has also been connected to the capacitor casing to increase the efficiency of the evacuating process. After the capacitors have been subjected to the sub-atmospheric pressure and elevated temperature for a period of about four days, the dielectric liquid is admitted into the casing to impregnate the dielectric material of the capacitor. In normal processing the capacitors are permitted to soak under vacuum and at a temperature in the range of 120.degree. F. to 190.degree. F. for a period up to several days to thoroughly impregnate the dielectric layers with the dielectric liquid. After this soaking period, the capacitors are sealed and are ready for other manufacturing processes.
Recently, polymeric film, such as polypropylene or polyethylene, has had increased use as the dielectric material in electrical apparatus. Conventional processing techniques cannot be successfully applied to capacitors having all polymeric film as the dielectric material, because the elevated temperatures cause an expansion of the molecular structure of the polymeric film and the expansion adversely effects the diffusion of the liquid dielectric into the polymeric film.